Transfection agents, such as lipid aggregates comprising cationic lipid components have been used to deliver large anionic molecules, such as nucleic acids, into certain types of cells. See Felgner et al., Nature 337:387-388 (1989); Proc. Natl. Acad. Sci. USA 84:7413 (1987). These agents are not, however, universally effective in all cell types. In many cases, cationic lipids alone are not effective or are only partially effective for transfection. Moreover, these methods do not work for all cell types, often require relatively complex protocols and are inconvenient. It is apparent, therefore, that new and improved methods for introducing macromolecules, and particularly nucleic acids, into cell, are greatly to be desired. In particular, improved methods for introducing nucleic acids into a wider variety of cells, and particularly into primary cells, are greatly to be desired.